The present invention relates generally to apparatus for coating a web material, such as paper or board web, and more particularly, to apparatus for coating a web including a coating nip formed between a rotating counter roll and a smoothing or coating member or between a pair of smoothing members, through which nip the web to be coated is passed with a coating substance being applied to the web prior to passing through the nip.
Prior art relevant to the present invention includes CA Patent Nos. 563,750, 689,123, and 1,184,758, FI Patent Application No. 83 3464, and SE Patent Nos. 8205805 and 8205807. Reference is also made to a paper published in paper 201 (1984) wherein a "Short-Dwell Coater" (a trademark of Beloit Corporation) is described.
Various arrangements are known for coating one or both sides of webs, such as paper webs. For example, roll coating devices, blade coating devices and blade-rod coating devices are known for single sided and two-sided coating of paper webs.
In conventional roll coating devices, the web passes through one or more coating nips formed by two opposed rolls with one or more pools of size or coating substance being formed in connection with the nip or nips. In blade coating devices, a counter roll and a coating blade or, alternatively, a pair of blade devices, form the coating nip. In a blade-rod coating device, a counter roll and a blade rod form the coating nip in connection with which the coating substance is applied to the web.
The running speeds of web coating devices have increased in recent years. Speeds as high as 1500 to 1800 m/min are reached during the web coating operation. Moreover, as the widths of paper webs have increased, the precision and durability requirements imposed on the coating devices, i.e., on the coating blades or blade-rods of the coating devices, have become even more exacting.
In this connection, a problem has arisen in the operation of conventional coating arrangements in that coating blades become worn quite rapidly, e.g., requiring replacement after operating for only about 8 to 24 hours. On the other hand, blade coating devices are advantageous in that the transverse distribution of the quantity of coating substance applied to the web can be adjusted quite precisely since the blade is sufficiently flexible in the direction transverse to the direction of movement of the web, that is, in the width dimension of the web.
On the other hand, the rods in blade-rod coating devices are worn less rapidly than blades, requiring replacement only after one to two weeks of operation which is considered satisfactory. However, a drawback of coating arrangements using coating rods is that it is generally not possible to adjust the distribution of the coating substance on the web with the precision desired since the coating rod mounted in its support is so rigid that it will not yield to a sufficient extent in order, for example, to accommodate variations in thickness of the web to be coated.